The present invention relates to an active liquid crystal display panel and, more particularly, to an active liquid crystal display panel which has a construction that mitigates the severity of the defect of a defective pixel.
A description will be given first, with reference to FIGS. 1 and 2, of a prior art example of an active liquid crystal display panel. Reference numeral 11 denotes a transparent substrate, over which pixel electrodes 15 are arranged in a matrix form. Adjacent the pixel electrodes 15 there are formed on the transparent substrate 11 TFT's (this film transistors) 16 which serve as switching elements. Reference numeral 18 indicates gate lines each formed along one of rows of the pixel electrodes 15. Reference numeral 19 denotes source lines each formed along one of columns of the pixel electrodes 15. The TFT's 16 each have its gate connected to the corresponding gate line 18 and have its source connected to the corresponding source line 19. The drain of each TFT 16 is connected to the corresponding pixel electrode 15. Reference numeral 17 denotes a common electrode formed over the interior surface of a transparent substrate 12 disposed opposite the transparent substrate 11 with liquid crystal 14 sealed in the space defined therebetween. The gate lines 18 and the source lines 19 are both selectively driven and only that one of the TFT's 16 whose gate and source are both driven by the selectively driven gate and source lines conducts, applying a voltage to the pixel electrode 15 corresponding to the conducting TFT 16. As a result, the optical condition of the liquid crystal 14 between the energized pixel electrode 15 and the common electrode 17 varies.
In FIG. 3 there is shown in section one TFT 16 on the transparent substrate 11 and its vicinity in the case where the TFT 16 is a top gate type one. Over an area of the transparent substrate 11 corresponding to the position where to form the TFT 16 there is provided a light intercepting or shielding layer 21 as of chromium (Cr) or molybdenum (Mo), which is covered with a first insulating layer 22 deposited almost all over the inside surface of the substrate 11. On the top of the first insulating layer 22 there are provided the pixel electrodes 15 and the source lines 19 made of a transparent conductor such as ITO and a semiconductor layer 23 which bridges the gap between the pixel electrode 15 and the source line 16 to form the TFT 16. The pixel electrodes 15, the source line 19 and the semiconductor layer 23 are covered with a second insulating layer 24 of silicon nitride deposited as a gate insulating layer almost all over the inside surface of the substrate 11. On the top of the second insulating layer 24 a gate electrode 25 facing the semiconductor layer 23 and the gate line 18 connected to the electrode 25 are simultaneously formed of the same metal, for instance, aluminum (Al). The shielding layer 21 is provided to prevent that a photoelectric effect is produced by the incidence of light to the semiconductor layer 23.
The transparent substrates 11 and 12, which constitute the liquid crystal display panel, are covered over the entire areas of their outside surfaces with polarizing film 10a and 10b. For example, when the liquid crystal 14 is a 90-degree twisted nematic liquid crystal, the liquid crystal display panel provides a display in a normally white or black mode, depending on whether the directions of polarization of the polarizing films 10a and 10b are perpendicular or parallel to each other. In the case where the polarizing films 10a and 10b are deposited with their directions of polarization perpendicular to each other, light incident to the liquid crystal 14 through the one polarizing film is polarized through 90 degrees when no voltage is applied to the liquid crystal 14, and hence the light is permitted to pass through the other polarizing film. On the other hand, when a voltage is applied to the liquid crystal 14, the light incident thereto through the one polarizing film is not polarized, and hence is intercepted by the other polarizing film.
In the active liquid crystal display panel which operates in the normally white mode as mentioned above, the display state of every pixel corresponding to the pixel electrode supplied with a voltage is changed from light transmitting state to light intercepting state. If a pixel develops a defect by which no voltage is applied across the corresponding pixel electrode and the common electrode at all times, the pixel will become a white-defect pixel which is always light transparent. In a color liquid crystal display, in particular, the white detect is far more noticeable as compared with a block defect that inhibits the passage of light through the defective pixel, and hence the permissible number of white defects in the liquid crystal display panel is smaller than the permissible number of black defects. Accordingly, the yield rate of fabrication of normally white mode color liquid crystal display panels is inevitably low. Also in black and white liquid crystal display panels, it is required, according to the purpose of use, user's liking or the like, that the number of white and black defects be smaller than predetermined numbers for the normally white mode and the normally black mode, respectively. In this instance, the yield rate of fabrication of the liquid crystal display panels can be improved by changing the white defect to the black one in the normally white mode panel and the black defect to the white one in the normally black mode panel.
To alleviate or suppress the deterioration of the display quality caused by the white detect in such a normally white mode active liquid crystal display panel, there is proposed, in U.S. Pat. No. 5,121,236 issued after the priority date of this application, a technique of changing the white defect to a black one by shorting the pixel electrode of the defective pixel to the gate or source line and supplying it with a potential during the operation of the liquid crystal display panel. According to the U.S. patent, a short-circuit metal layer of, for example, chromium (Cr) or molybdenum (Mo) is provided on a transparent substrate over and an area where it underlaps adjacent or opposed marginal portions of the pixel electrode and the source line with the insulating layer sandwiched therebetween. Welding metal pads, which are also formed of chromium or molybdenum, are provided on those areas of the marginal portions of the pixel electrode and the source line at positions where they overlap the short-circuit metal layer. If the pixel is a white-defect pixel, then a laser beam is applied to the overlapping portions of its welding metal pads and short-circuit metal layer to fuse them and destroy those areas of the insulating layer underlying them, thereby forming a short circuit between the pixel electrode of the defective pixel and the source line through the welding metal pads and the short-circuit metal layer thus fused together. Once the defective pixel electrode is connected to the source line, the potential that is normally applied to the source line is provided also to the defective pixel electrode. As a result, the white-defect pixel becomes a black-defect pixel.
In the above-mentioned U.S. patent, since the welding metal pads and the short-circuit metal layer are formed of a material of a high melting point, such as chromium or molybdenum, it is necessary that the regions to be irradiated by the laser beam be heated to an appreciably high temperature enough to melt the metal pads and the short-circuit metal layer and destroy the insulating layer so that are fused together. This incurs the possibility that the metal pads and an insulating layer overlying them may sometimes peel off and gets turned up, producing a short circuit between them and the common electrode on the other of the pair of opposed substrates.